Three factors have inhibited improvements in the gain, power output and bandwidth of monolithic distributed amplifiers: input line attenuation, output line attenuation, and the dynamic (linear) range of the input signal. The primary loss mechanisms of the input and output lines are the transistor loadings rather than the (microstrip) interconnecting line losses. Advanced transistor design is an important way in which distributed amplifier performance can be improved, but is not the only means available.
A typical conventional distributed amplifier circuit consists of periodically spaced field-effect transistors (FETs) which are connected by electrically short, high impedance microstrip lines. The design of such conventional amplifiers is generally discussed in, for example, the article by J. B. Beyer, et al., "MESFET Distributed Amplifier Design Guidelines" IEEE Trans. Microwave Theory Tech., Vol. MTT-32, March, 1984, pp. 268-275, which is incorporated herein by reference. A distributed amplifier may be qualitatively described as a set of artifical input and output transmission lines which are coupled by (FET) transconductances. A schematic view of a typical conventional distributed amplifier having an arbitrary number of sections `n` is shown in FIG. 1. The amplifier has an input port 10, an output port 11, an input line 14 consisting of a series of high impedance microstrip artificial transmission lines 15, an output line 17 composed of a series of high impedance microstrip lines 18, an image matching port 20 on the output line having an image matching impedance and DC drain bias source 21 connected to ground, an image matching port 23 on the input line having an image matching impedance and DC gate bias source 24 connected to ground, and a series of field effect transistors (FETs) 26 connected in a common-source configuration. The drains of each FET are connected by microstrip lines 28 to junction nodes between the microstrips 18 of the output line, and the gates are connected by connecting lines 29 to junction nodes between the input line microstrips 15. The terminations 21 and 24 are standard and well known in the art, and various gate and drain biasing circuits may be used. See, e.g., the biasing and termination circuits shown in U.S. Pat. Nos. 4,543,535, 4,595,881, and 4,486,719. The conventional distributed amplifier of FIG. 1 and the present invention are illustrated utilizing microstrip transmission lines although, of course, lumped inductance and capacitance transmission lines may also be used.
Other analyses of the design fundamentals and trade-offs of distributed amplifier designs are given in R. C. Becker, et al., "On Gain-Bandwidth Product for Distributed Amplifiers," IEEE Trans. Microwave Theory Tech., Vol. MTT-34, June, 1986, pp. 736-738; J. B. Beyer, et al., "Wideband Monolithic Microwave Amplifier Study," University of Wisconsin-Madison, Dept. ECE, Report No. ECE-83-6 1983. A conclusion drawn from the foregoing papers is that the input (gate) and output (drain) line attenuation parameters A.sub.g and A.sub.d control the gain and bandwidth of the conventional distributed amplifier. The maximum DC gain is related to the value of A.sub.d, i.e., inversely proportional to the function sinh [A.sub.d (0 Hz)/2]. Lower output loss FETs, higher image impedances, and/or higher gain FETs are necessary to extend the DC gain limits in the conventional distributed amplifier design. At higher frequencies, the impact of the input line attenuation A.sub.g is more pronounced. A reduction of A.sub.g would allow higher distributed amplifier operating frequencies.